The retention of a monolithic substrate in a catalytic converter housing presents a difficult technical problem because of the high temperatures and pulsating flow conditions in the exhaust gas stream from an automotive engine. And these conditions are especially severe for converters mounted close to, or directly as part of the exhaust manifold.
Prior attempts at solving this problem have used retaining lips at the ends of the housing to capture the substrate at the peripheral end edges thereof. While this has proven to be generally satisfactory for ceramic substrates, such is not the case with monolithic metal foil substrates. Typically, such monolithic metal foil substrates are formed by either spirally wrapping the foil, folding a continuous sheet or stacking individual sheets. Where the layers are formed by either stacking or folding, each layer is effectively trapped by the conventional retaining rings, however, in the case where the layers are formed by spiral wrapping, it has been found that the layers inward of such retaining rings may telescope due to the pressure drop forces. Moreover, regardless of how the metal foil layers are formed, it has been found that the foil may shear at the retaining lip-to-foil interface and may also crack in both the longitudinal and transverse directions. Furthermore, there may occur retainer lid deformation as a result of vibrations and flow pulsations acting on the substrate which in turn impacts the lip and can result in substrate movement whether the lip is wide or narrow. As a result, it has been the practice to weld or otherwise attach the layers together such as with staples or integral barbs and to also add further retention means such as cross-wise pieces or bars at the ends of the foil layers.
It is also known to provide retention of a monolithic substrate, either ceramic or metal, by employing one or more solid pins inserted through the monoliths. These pins pass through the substrate across the housing and are attached to the latter to resist axial pressure drop forces tending to force the substrate out of the housing and to prevent those layers such as in a spiral wrap from telescoping. Typically, these pins are welded at their ends to opposite sides of the housing and serve either as a supplementary restraint or in lieu of retainer lips to eliminate the shearing effect of the latter. However, it has been found that differential thermal expansion can cause distortions when such pins are used. In use, the pin is exposed directly to exhaust gas and consequently is heated to a higher temperature than the housing because the pin has limited access to a heat sink. In contrast, the housing is exposed directly to ambient air flow and associated cooling heat transfer.
Because the pin is heated to a temperature higher than that of the housing, thermal expansion causes the pin length to be greater than the cross-wise dimension of the housing. This thermal growth mismatch must be accommodated, either by thermal strain within the parts, or by thermal distortion of the parts. Thermal distortion of either the housing or the pin is undesirable because parts no longer fit so leakage or looseness, or both can result.